Escherichia coli O157:H7 causes bloody diarrhea and potentially fatal systemic sequelae in humans. Cattle are most frequently identified as the primary source of infection, and E. coli O157:H7 generally colonizes the gastrointestinal tracts of cattle without causing disease. In this study, persistence and tropism were assessed for four different E. coli O157:H7 strains. Experimentally infected calves shed the organism for at least 14 days prior to necropsy. For the majority of these animals, as well as for a naturally colonized animal obtained from a commercial beef farm, the highest numbers of E. coli O157:H7 were found in the feces, with negative or significantly lower levels detected in lumen contents taken from the gastrointestinal tract. Detailed examination demonstrated that in these individuals the majority of tissue-associated bacteria were adherent to mucosal epithelium within a defined region extending up to 5 cm proximally from the recto-anal junction. The tissue targeted by E. coli O157:H7 was characterized by a high density of lymphoid follicles. Microcolonies of the bacterium were readily detected on the epithelium of this region by immunofluorescence microscopy. As a consequence of this specific distribution, E. coli O157:H7 was present predominately on the surface of the fecal stool. In contrast, other E. coli serotypes were present at consistent levels throughout the large intestine and were equally distributed in the stool. This is a novel tropism that may enhance dissemination both between animals and from animals to humans. The accessibility of this site may facilitate simple intervention strategies.Enterohemorrhagic Escherichia coli (EHEC) has emerged as an important cause of human intestinal disease in developed countries over the past 20 years. In addition to bloody diarrhea, intestinal infection can lead to potentially fatal systemic sequelae resulting from the activity of Shiga toxins. The majority of these infections are caused by E. coli O157:H7 (21,26). This serotype has been frequently isolated from cattle feces, and most human EHEC O157:H7 infections originate, either directly or indirectly, from this source (5, 8). A key step in protecting public health is to know how and where the bacterium persists in this major animal reservoir. Until now, no defined site of colonization by E. coli O157:H7 in the bovine gastrointestinal tract (GIT) has been described, beyond an affinity for the large intestine (17).Enteropathogenic E. coli (EPEC) and most EHEC strains are known to carry a locus of enterocyte effacement (LEE) pathogenicity island (24). This locus encodes a type III secretion system that mediates attachment to mucosal epithelial cells. Injection of effector proteins results in intimate attachment and characteristic attaching and effacing (A/E) lesions dependent on intimin and Tir (translocated intimin receptor) (16,22). E. coli O157:H7 intimately attaches to a variety of cell types and tissues in vitro, and a few studies have demonstrated that E. coli O157:H7 can form A/E lesions ...

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Identification of the relative importance of within-and betweenhost variability in infectiousness and the impact of these heterogeneities on the transmission dynamics of infectious agents can enable efficient targeting of control measures. Cattle, a major reservoir host for the zoonotic pathogen Escherichia coli O157, are known to exhibit a high degree of heterogeneity in bacterial shedding densities. By relating bacterial count to infectiousness and fitting dynamic epidemiological models to prevalence data from a cross-sectional survey of cattle farms in Scotland, we identify a robust pattern: Ϸ80% of the transmission arises from the 20% most infectious individuals. We examine potential control options under a range of assumptions about within-and betweenhost variability in infection dynamics. Our results show that the within-herd basic reproduction ratio, R 0, could be reduced to <1 with targeted measures aimed at preventing infection in the 5% of individuals with the highest overall infectiousness. Alternatively, interventions such as vaccination or the use of probiotics that aim to reduce bacterial carriage could produce dramatic reductions in R 0 by preventing carriage at concentrations corresponding to the top few percent of the observed range of counts. We conclude that a greater understanding of the cause of the heterogeneity in bacterial carriage could lead to highly efficient control measures to reduce the prevalence of E. coli O157.bacterial count ͉ core groups ͉ super shedder ͉ superspreading ͉ targeted control T he role of heterogeneous infectiousness on the course of disease outbreaks was highlighted during the recent severe acute respiratory syndrome outbreak (1), in which a few individuals were responsible for a disproportionate number of transmission events. Awareness of heterogeneities in transmission dynamics can be important for the effective implementation of disease control measures and can lead to efficient targeting of interventions at a subset of the population (2-5). Factors that might lead to such heterogeneities include variability in infectiousness, exposure, genetic susceptibility, contact rates, and behavior (6-10). Quantifying their impact on the transmission dynamics can be achieved through direct methods, such as contact tracing and outbreak reconstruction (1, 11), or indirectly through their effect on the distribution of infected cases (12).Escherichia coli O157 is an important zoonosis with a known reservoir in cattle (13,14). Prevalences of infection are generally low, usually reported to be Ͻ10% of animals carrying the pathogen (14). Typically, however, the distribution of prevalences is highly skewed (15); at any one time, shedding is not detected in the majority of cattle groups, but a small proportion of groups contains high numbers of individuals shedding bacteria in their feces.The range of prevalences of an infectious agent in a small population is expected to be influenced both by stochasticity and underlying heterogeneities in the transmission dynamics. In a recent a...

Escherichia coli O157 infections are the cause of sporadic or epidemic cases of often bloody diarrhea that can progress to hemolytic uremic syndrome (HUS), a systematic microvascular syndrome with predominately renal and neurological complications. HUS is responsible for most deaths associated with E. coli O157 infection. From March 2002 to February 2004, approximately 13,000 fecal pat samples from 481 farms with finishing/ store cattle throughout Scotland were examined for the presence of E. coli O157. A total of 441 fecal pats from 91 farms tested positive for E. coli O157. From the positive samples, a point estimate for high-level shedders was identified using mixture distribution analysis on counts of E. coli O157. Models were developed based on the confidence interval surrounding this point estimate (high-level shedder, greater than 10 3 or greater than 10 4 CFU g ؊1 feces). The mean prevalence on high-level-shedding farms was higher than that on low-levelshedding farms. The presence of a high-level shedder on a farm was found to be associated with a high proportion of low-level shedding, consistent with the possibility of a higher level of transmission. Analysis of risk factors associated with the presence of a high-level shedder on a farm suggested the importance of the pathogen and individual host rather than the farm environment. The proportion of high-level shedders of phage 21/28 was higher than expected by chance. Management-related risk factors that were identified included the type of cattle (female breeding cattle) and cattle stress (movement and weaning), as opposed to environmental factors, such as water supply and feed.Verocytotoxin-producing Escherichia coli (VTEC), such as E. coli O157, is an important zoonotic agent with worldwide distribution. E. coli O157 may cause sporadic or epidemic cases of often bloody diarrhea that can progress to hemorrhagic colitis, thrombotic thrombocytopenic purpura, and hemolytic uremic syndrome (HUS) (21). HUS is a systematic microvascular syndrome that is initiated by secreted shiga toxins, with predominately renal and neurological complications, which are responsible for most deaths associated with E. coli O157 infection, particularly among elderly patients (22). Infection with E. coli O157 is a leading cause of acute renal failure in children (8). The incidence of E. coli O157 infection in Scotland is substantially higher than elsewhere in Great Britain (28) Healthy cattle shed E. coli O157 in their feces (9,17,35), and this pathogen is present in most cattle operations (48). Cattle are the main reservoir host for E. coli O157 and other VTEC in the developed world (1) and play a significant role in the epidemiology of human infections (13). Outbreaks are attributed to consumption of contaminated food and water, animal contact, and person-to-person transmission (51). However, case control studies of sporadic infections, which account for the majority of cases of E. coli O157 infection in Scotland, have indicated direct contact with animals, their feces, and/...

Escherichia coli O157:H7 is an important cause of diarrhea, hemorrhagic colitis, and potentially fatal human illness. Cattle are considered a primary reservoir of infection, and recent experimental evidence has indicated that the terminal rectum is the principal site of bacterial carriage. To test this finding in naturally colonized animals, intact rectum samples from 267 cattle in 24 separate lots were obtained immediately after slaughter, and fecal material and mucosal surfaces were cultured for E. coli O157 by direct and enrichment methods. Two locations, 1 and 15 cm proximal to the recto-anal junction, were tested. In total, 35 animals were positive for E. coli O157 at at least one of the sites and 232 animals were negative as determined by all tests. The frequency of isolation and the numbers of E. coli O157 cells were higher at the site closer to the recto-anal junction, confirming our previous experimental findings. We defined low-and high-level carriers as animals with E. coli O157 levels of <1 ؋ 10 3 CFU g ؊1 or <1 ؋ 10 3 CFU ml ؊1 and animals with E. coli O157 levels of >1 ؋ 10 3 CFU g ؊1 or >1 ؋ 10 3 CFU ml ؊1 in feces or tissues, respectively. High-level carriage was detected in 3.7% of the animals (95% confidence interval, 1.8 to 6.8%), and carriage on the mucosal surface of the terminal rectum was associated with high-level fecal excretion. In summary, our results support previous work demonstrating that the mucosal epithelium in the bovine terminal rectum is an important site for E. coli O157 carriage in cattle. The data also support the hypothesis that high-level fecal shedding (>1 ؋ 10 3 CFU g of feces ؊1 ) of enterohemorrhagic E. coli O157 results from colonization of this site.